A lack of fossil energy and a necessity for a reduction in air pollutants and carbon dioxide (CO2) emissions may contribute to an increasing interest in generation of a heat source and use of technology using a substitute, for example, renewable energy, without pollutants.
Soil has nearly a consistent temperature although being approximately 10 meters (m) to 20 m below a ground surface. The temperature increases when a ground depth increases. A natural temperature gradient of the soil is approximately 0.03 Kelvin (K)/m relative to the depth, and the temperature is determined by a heat flux inside the earth.
In general, recovery of a heat source may be performed through circulation of a heat transfer medium. Here, a ground surface heat exchanger may be vertically installed 50 m to 350 m in depth and a deep ground heat exchanger may be installed 1,000 m to 5,000 m in depth.
In general, a traditional ground heat exchanger may be designed as a U-shaped tube exchanger including a U-shaped tube, and a heat transfer medium in such an exchanger may flow from a surface to a bottom of a hole of the exchanger at one point of the tube. That is, the heat transfer medium may flow from top to bottom.
At another point of the tube, a heated circulating heat transfer medium may flow from a borehole base to the surface. That is, the heat transfer medium may flow from bottom to top. When ascending, the heat transfer medium may release some of accumulated heat energy permanently towards a heat transfer medium circulating downwards at a neighboring point of the tube and towards a colder soil surrounding the tube. Due to such a heat release, exergetic efficiency of the U-shaped tube heat exchanger may not be relatively high.
To improve such an efficiency, a coaxial ground heat exchanger may be used. In a case of a coaxial ground heat exchanger in an annular outer gap, for example, a loop-shaped outer gap, a heat transfer medium may flow from a ground surface to a borehole base, and may obtain sensitive heat energy present in soil and then flow back towards the surface through a central core tube.
The heat transfer medium that is injected through an injection hole may absorb heat therearound while descending and thus be heated, and reach a highest temperature when arriving at a bottom of a ground heat hole. When the heat transfer medium is deprived of heat by the surroundings while ascending via a production hole, the heat transfer medium with a reduced temperature may be discharged out of the ground.
In such a coaxial ground heat exchanger when heat transfer of the central core tube is blocked, the exergetic efficiency may considerably increase.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention as defined in the claims is to be bound.